1. Field of the Invention
The invention relates to an implant and to a method of manufacturing an implant packed in a sterile manner.
2. Description of the Related Art
Ultra-high molecular weight polyethylene (UHMWPE) is a material widespread in orthopaedic endoprosthetics which has proven itself as a sliding partner with respect to metallic and ceramic materials. A problem with these material pairings is the wear unavoidable in use and the particle-induced osteolysis triggered thereby which in the worst case can result in a loosening of the implant.
A suitable measure for the optimization of the UHMWPE with the aim of reducing the arising wear is high cross-linking. This is achieved by electron radiation or gamma radiation and a heat treatment of the material, whereby the ultra-high molecular weight polyethylene is made wear resistant and given long term stability. The high cross-linking takes place by a cross-linking of the UHMWPE molecule chains and by a de-saturation of the free radicals.
The sterilization method of gamma radiation usual in orthopaedic endoprosthetics cannot be used for highly cross-linked UHMWPE, because unwanted free radicals would again thereby arise. These free radicals promote an oxidization effect and an embrittlement of the associated material. Highly cross-linked UHMWPE is therefore not sterilized with ionizing radiation, but with gas, for example with ethylene oxide or gas plasma.
With implant components which consist exclusively of highly cross-linked polyethylene, satisfactory results are achieved by means of high cross-linking and heat treatment of the UHMWPE blank, subsequent mechanical processing, packaging and gas sterilization.
However, implants can consist of a plurality of elements put together during the manufacturing process. With such composite implants, the problem of effective sterilization arises. A sterilization with gamma rays would cover the whole implant and thus oxidize elements made of UHMWPE. A gas sterilization, in contrast, only acts on all those surfaces of the implant which are accessible to the gas during the sterilization. To also make surfaces covered by additional elements accessible to the sterilizing gas, it is known in this connection to carry out the connection of implant elements with play. However, such play is problematic on the basis of clinical experience, since micro-movements of implant elements made possible in this manner are under suspicion of producing unwanted wear and of countering a permanent ingrowth of the bone.